Automated assessment of peripheral vascular condition

ABSTRACT

An automated assessment of a patient&#39;s peripheral vascular condition includes using a pulse oximeter to generate at least a perfusion index relative to a limb or digit of the patient. Pressure is applied to the limb or digit, and while increasing or decreasing the pressure, a change in the perfusion index is determined. The change is indicative of a cessation of blood flow or a return of blood flow in the limb or digit. A systolic blood pressure is thereafter determined based on the pressure applied at the time of cessation of blood flow or the return of blood flow in the limb or digit. Using a pulse oximeter to generate a perfusion index may include transmitting light into the limb or digit, detecting light that was transmitted into tissue in the limb or digit, and calculating the perfusion index based on the light transmitted through the tissue.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 13/791,806,filed Mar. 8, 2013, the disclosure of which is incorporated herein byreference.

TECHNICAL FIELD

The present application relates to an automated medical apparatus andmethods for assessing a peripheral vascular condition of a patient.

BACKGROUND

Peripheral Arterial Disease (PAD) is an international epidemic of asignificant size. For example, in the United States, an estimated 12-20million people are affected. Closely related to diabetes mellitus, PADis the peripheral vascular component of the systemic diseaseatherosclerosis which also causes heart attack and stroke. The incidenceof PAD is increasing rapidly. Once PAD becomes critical limb ischemia,the 5-year mortality is worse than cancer, heart attack, and stroke,with 69% mortality as illustrated in FIG. 1. People with coronary heartdisease and/or carotid disease who also have PAD have 3 to 6 times therisk of a coronary event or stroke as those without PAD.

PAD is called a multi-vessel disease when it is in the peripheralarteries of the legs, the carotid arteries of the neck, and/or thecoronary arteries of the heart. Because 90% of people with PAD have nosymptoms and are unaware that they have the disease, it often goesunnoticed until they have either a cardiac event or cerebrovascularaccident (CVA), or if it progresses until there is critical limbischemia with severe leg pain on exertion, which can lead to amputation.Due to the “invisibility” of PAD, it is difficult to accuratelydetermine how many people have PAD.

The American Heart Association, the American College of Cardiology, theAmerican Diabetes Association, the American College of Physicians, andothers, have generated a series of PAD-related guidelines that wererevised in January 2011. They believe it is imperative that primary carephysicians perform screening for PAD, especially with diabetic patients.Because of the increasing incidence and lower age of people developingPAD, they also reduced the recommended screening age.

A typical patient who should be screened for PAD often presents with acluster of diseases such as diabetes, hypertension, dyslipidemia,coronary artery disease (CAD), and CVA or stroke. Only about 10% of allpatients with PAD are symptomatic.

The definitive screening exam for PAD is an Ankle Brachial Index or ABI,and for patients with calcified vessels in their legs, a Toe BrachialIndex or TBI. These exams have traditionally been performed by vascularspecialists and vascular surgeons using a continuous wave (CW),bi-directional (zero-crossing) Doppler. The ABI is a non-invasive testthat compares the highest systolic brachial pressure of the two arms tothe highest ankle pressure in each leg by dividing the ankle pressure bythe brachial pressure. The resulting number is the Ankle Brachial Index.A person with two legs has two ABI readings. A normal ABI is 1.0. Anumber below 0.99 shows the presence of decreased arterial blood flowdue to PAD, with the disease severity increasing as the index decreases.Occasionally a diabetic patient will have very stiff vessels that areincompressible due to calcification of the arterial walls, and he willhave an ABI greater than 1.3. To determine perfusion to the foot, thesepatients should also have a TBI performed, since vessels of the toe donot normally calcify like those of the legs.

Presently, a CW, bi-directional Doppler is recognized as the appropriateinstrument for performing ABI measurements. ABI measurements taken usinga CW Doppler are reimbursable by the Centers for Medicare and MedicaidServices (CMS). In the United States, Doppler is currently recognized asthe gold standard for performing the ABI.

It is important in clinical practice to measure blood pressures for theABI or TBI using the same technology and technique on the patient sothat the pressures obtained from multiple locations on the patient canbe directly compared. Other techniques of measuring blood pressure inthe brachial arteries are also used; however, they have been shown to beunreliable and inaccurate when applied to the ankle arteries.

Most primary care physicians, unlike the vascular specialists currentlyperforming ABI exams, have little or no recent experience using aDoppler and may feel uncomfortable relying on their own expertise toperform, analyze, and diagnose disease using this instrument. They arewary of sending a false-positive patient to a vascular surgeon andexperiencing the embarrassment when, upon ultrasound scanning performedby the vascular surgeon, the patient is told there is no disease. Theyare also wary of the reverse when a false-negative results in missing adiseased limb that needs follow-up.

Through experience, vascular clinicians are able to find the bestlocation over an artery where a clean waveform can be achieved withexcellent Doppler sound, and by inflating a vascular cuff, obtain anaccurate systolic pressure measurement. This involves specializedtechnique, applying the right size cuff, holding the Doppler sensor atthe correct angle in regard to the arterial walls and subsequent bloodflow, recognizing what is heard, understanding how to inflate anddeflate the cuff correctly, and finally calculating the index for eachleg.

As mentioned above, ABI and TBI examinations are reimbursable in theUnited States provided the procedure meets CMS requirements. If aresultant ABI is abnormally high (>1.3), then toe pressures aresubstituted for the ankle pressures since, unlike the legs, the toes donot calcify and are compressible. The pressures in the toes are measuredby putting a pressure cuff on the toe and obtaining aphotoplethysmograph waveform (PPG) distally on the pad of that toe. Theresultant TBI reflects an accurate state of the arteries in thepatient's legs. PPG is the standard instrument used to determine toepressures in the United States because it is more accurate and easier touse than a CW Doppler. It is most difficult to perform a toe pressureusing a Doppler since there is no major artery near the surface of thepad on the bottom of the toe, whereas the PPG transducer can be attachedto the pad of the toe with Velcro, a clip, or tape, and can detect bloodflow via the pulsating arterioles.

As with Doppler, photoplethysmography is not commonly practiced inprimary care physician offices, and as such, the clinicians aretypically not familiar with normal waveforms and the associatedreadings.

Providing a physician or other medical clinician with an easy-to-use,accurate, and reliable means of performing an ABI and/or TBI, eithersingularly or in conjunction with a bi-directional Doppler, can providereassurance that the results are accurate, especially when the actualmeasurements and calculations are performed automatically and areverifiable.

SUMMARY

Described herein is an automated method of assessing a peripheralvascular condition of a patient. In at least one embodiment, undercontrol of a computing device, the method includes using a pulseoximeter to generate a perfusion index (PI) relative to a limb or digitof the patient. Pressure is applied to the limb or digit of the patient,and while increasing or decreasing the pressure, a change in theperfusion index relative to the limb or digit is determined. The changein the perfusion index is indicative of a cessation of blood flow or areturn of blood flow in the limb or digit. A systolic blood pressure inthe limb or digit is determined based on the pressure applied at thetime of cessation of blood flow or the return of blood flow in the limbor digit. The resultant systolic pressure determinations are used tocalculate ABI and TBI measurements.

In various embodiments, a pressure cuff is used to compress the limb ordigit of the patient. The pressure cuff may inflate or deflate at acontrolled rate.

In various embodiments, a pulse oximeter generates a perfusion index bytransmitting light into the limb or digit, detecting light that wastransmitted through tissue in the limb or digit, and calculating theperfusion index relative to the limb or digit based on the lighttransmitted through the tissue. Determining a change in the perfusionindex indicative of a cessation of blood flow in the limb or digit mayinclude identifying when a decrease in the perfusion index occurs.Alternatively (or additionally), a change in the perfusion indexindicative of a return of blood flow in the limb or digit may bedetermined by identifying when an increase in the perfusion indexoccurs.

In various embodiments, an assessment index such as an ABI may becalculated by dividing a systolic blood pressure determined in an ankleof the patient by a systolic blood pressure determined in an arm of thepatient. Alternatively (or additionally), an assessment index such as aTBI may be calculated by dividing a systolic blood pressure determinedin a toe of the patient by a systolic blood pressure determined in anarm of the patient.

In various embodiments, while increasing or decreasing the pressureapplied to the limb or digit, the above-described methods may furtherinclude monitoring a change in oxygen saturation (SpO₂) of the blood inthe limb or digit indicative of a cessation of blood flow or a return ofblood flow in the limb or digit. A systolic blood pressure in the limbor digit is determined based on the pressure applied when the cessationof blood flow or the return of blood flow occurred, as indicated by thechange in the perfusion index and by the change in oxygen saturation.

In various embodiments, while increasing or decreasing the appliedpressure, the above-described methods may include further monitoring avolumetric pulse waveform of the blood in the limb or digit for a changeindicative of a cessation of blood flow or a return of blood flow in thelimb or digit. A systolic blood pressure in the limb or digit isdetermined based on the pressure applied when the cessation of bloodflow or the return of blood flow occurred, as indicated by the change inthe perfusion index and by the change in the volumetric pulse waveform.

In various embodiments, while increasing or decreasing the appliedpressure, the above-described methods may further include monitoringoxygen saturation and a volumetric pulse waveform of the blood in thelimb or digit for a change indicative of a cessation of blood flow or areturn of blood flow. A systolic blood pressure in the limb or digit isdetermined based on the pressure applied when the cessation of bloodflow or the return of blood flow occurred, as indicated by the change inthe perfusion index, the change in oxygen saturation, and the change inthe volumetric pulse waveform.

Also described herein is an apparatus for assessing a peripheralvascular condition of a patient. In at least one embodiment, theapparatus may include a pressure cuff and a processor. The pressure cuffis configured to apply pressure to a limb or digit of the patient tocause cessation of blood flow in the limb or digit. The processor isconfigured to cause the pressure cuff to controllably apply or releasepressure to the limb or digit of the patient. The processor is furtherconfigured to monitor a perfusion index calculated by a pulse oximeterrelative to the limb or digit and determine a change in the perfusionindex indicative of a cessation of blood flow or a return of blood flowin the limb or digit. The processor determines a systolic blood pressurein the limb or digit based on the pressure applied by the pressure cuffwhen the cessation of blood flow or the return of blood flow occurred.

In various embodiments, the pressure cuff is configured to applypressure to the limb or digit by inflating and compressing the limb ordigit. The processor may be configured to cause the pressure cuff tocontrollably apply or release pressure by inflating or deflating thepressure cuff at a controlled rate.

In various embodiments, the apparatus may include one or more opticaldevices configured to transmit and detect light when attached to thepatient. The perfusion index is monitored using data generated by thepulse oximeter, the sensor of which includes one or more opticaldevices. The optical devices may be configured to transmit light intothe limb or digit and detect light transmitted through tissue in thelimb or digit, and the processor may be configured to monitor theperfusion index calculated by the pulse oximeter based on the lighttransmitted through the tissue.

In various embodiments, one or more optical devices may comprise anoptical source configured to transmit light at specific wavelengths anda photodiode configured to detect light transmitted at the specificwavelengths. One or more optical devices may comprise a pulse oximetersensor, and the processor may be configured to receive the perfusionindex from the pulse oximeter.

In various embodiments, the processor may further be configured tocalculate an assessment index such as an ABI by dividing a systolicblood pressure determined in an ankle of the patient by a systolic bloodpressure determined in an arm of the patient. Alternatively (oradditionally), the processor may be configured to calculate anassessment index such as a TBI by dividing a systolic blood pressuredetermined in a toe of the patient by a systolic blood pressuredetermined in an arm of the patient.

In various embodiments, while causing the pressure cuff to controllablyapply or release pressure to the limb or digit of the patient, theprocessor may be further configured to monitor oxygen saturation of theblood in the limb or digit for a change in oxygen saturation indicativeof a cessation of blood flow or a return of blood flow in the limb ordigit. A systolic blood pressure in the limb or digit is determinedbased on the pressure applied when the cessation of blood flow or thereturn of blood flow occurred as indicated by the change in theperfusion index and by the change in the oxygen saturation.

In various embodiments, while causing the pressure cuff to controllablyapply or release pressure to the limb or digit of the patient, theprocessor may be further configured to monitor a volumetric pulsewaveform of the blood in the limb or digit for a change indicative of acessation of blood flow or a return of blood flow in the limb or digit.A systolic blood pressure in the limb or digit is determined based onthe pressure applied when the cessation of blood flow or the return ofblood flow occurred as indicated by the change in the perfusion indexand by the change in the volumetric pulse waveform.

In various embodiments, while causing the pressure cuff to controllablyapply or release pressure to the limb or digit of the patient, theprocessor may be further configured to monitor oxygen saturation and thevolumetric pulse waveform of the blood in the limb or digit. A systolicblood pressure in the limb or digit is determined based on the pressureapplied when the cessation of blood flow or the return of blood flowoccurred as indicated by the change in the perfusion index, the changein the oxygen saturation, and the change in the volumetric pulsewaveform of the blood in the limb or digit.

The foregoing summary is provided to introduce a selection of conceptsin a simplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph showing the 5-year mortality of critical limb ischemiarelative to common cancers;

FIG. 2 is a general block diagram of an embodiment of an automateddiagnostic apparatus configured in accordance with the presentdisclosure;

FIG. 3 is a general flow diagram of algorithms operating within anautomated diagnostic apparatus as shown in FIG. 2;

FIG. 4 is a detailed block diagram of the automated diagnosticapparatus; and

FIG. 5 is a detailed flow diagram of algorithms operating within theautomated diagnostic apparatus.

DETAILED DESCRIPTION

FIG. 2 illustrates a general block diagram of an automated medicaldiagnostic apparatus 10 configured to assess a peripheral vascularcondition of a patient 12. In various embodiments, for example, theapparatus 10 uses automated algorithms to measure systolic bloodpressure of the patient 12. The apparatus 10 may also determineindicators of peripheral vascular health, such as an ankle brachialindex and/or a toe brachial index.

In at least one embodiment, the apparatus 10 includes an inflatablepressure cuff 14 that, in use, is fitted on a limb or a digit of apatient 12, who is the subject of testing. The pressure cuff 14 wrapsaround the limb or digit, and upon inflation and deflation, provides forstopping and allowing blood flow in the limb or digit depending on thepressure applied by the cuff.

A pressure transducer 16 directly connected to the pressure cuff 14 isconfigured to detect the mechanical pressure applied by the cuff to thelimb or digit of the patient 12. The detected mechanical pressure isconverted to electronic cuff pressure data inside the transducer. Asdescribed in further detail below, the pressure transducer 16 providesthe electronic cuff pressure data to a processor 22.

The apparatus 10 is further configured to receive signals and/or datafrom a pulse oximeter 18 that includes a pulse oximeter sensor 20 placedon the patient 12. For example, the pulse oximeter sensor 20 is placedon a limb or digit of the patient 12. In various embodiments, the pulseoximeter sensor 20 is an optical device configured to transmit andreceive light. The sensor 20 transmits optical signals through tissue ofthe patient 12 and converts received optical signals into electronicdata. The sensor 20 provides the electronic data to the pulse oximeter18 for processing, e.g., to calculate a perfusion index and/or otherpulse oximeter data. Alternatively, the sensor 20 provides electronicsignals to the pulse oximeter 18 and the pulse oximeter 18 converts thesignals into electronic data. The pulse oximeter 18 may also beconfigured to control operational aspects of the pulse oximeter sensor20 by issuing control signals to the sensor.

A clip sensor and a reflectance sensor are two common types of pulseoximeter sensor. A clip sensor clips onto a patient and transmits lightthrough the tissue to a receiver on the opposite side of the tissue.Clip sensors are designed to be placed on digits (fingers or toes).

A reflectance sensor has both the transmit diodes and the receivephotodiode on the same side of the tissue. The transmit diodes transmitlight through the tissue and the photodiode receives light that isreflected back through the tissue. Reflectance sensors can be placed ona flat surface of the body such as the forehead or over the navicularbone of the foot (top of the foot over the arch).

Although both transmission- and reflectance-based pulse oximeter sensorsexist, and suitable measurements can be performed using either type ofsensor, for convenience of description, a transmission-based pulseoximeter sensor 20 is discussed herein in greater detail.

It should be noted that a pulse oximeter sensor differs from a Dopplertransducer. For example, an optical source in the pulse oximeter sensor20 transmits a wide beam of specific wavelengths of light into a volumeor region of tissue, and a photodiode in the sensor 20 detectstransmitted or reflected light at the specific wavelengths from thatvolume or region of tissue. A Doppler sensor or probe, on the otherhand, transmits an ultrasound signal into a region of tissue, butbecause of its narrow beam, the Doppler instrument is used to measureblood velocity in specific vessels within that region. A pulse oximetersensor 20 has some similarities to a PPG sensor, but a pulse oximetersensor transmits and receives at least two different wavelengths oflight, whereas a PPG sensor transmits and receives only a singlewavelength.

Compared to a PPG, a pulse oximeter is the preferred choice formeasuring peripheral vascular blood pressures because a pulse oximeterprovides numeric data as well as waveform output. Such numeric data mayinclude oxygen saturation (SpO₂) and perfusion index (PI) data, whilethe waveform output from a pulse oximeter may include those twoparameters and a volumetric pulse waveform. Sophisticated digital signalprocessing may be used to generate the numeric data. A PPG, on the otherhand, provides only a volumetric pulse waveform output, so evaluation ofthe output is typically performed visually, which is not nearly asaccurate as a numeric evaluation. Moreover, the sensitivity of currentlyavailable pulse oximeters far exceeds that of PPG in terms of obtainingaccurate data at low perfusion and in the presence of patient motion.Pulse oximetry parameters can be correlated with each other in terms ofperfusion, oxygenation, and volumetric pulse waveform, which can befurther used to validate a systolic pressure measurement as describedherein. This type of correlation would not be possible with PPG alone.

Photoplethysmography (PPG) is almost exclusively found in vascularspecialty centers where peripheral vascular testing is traditionallyperformed. PPG is not found in other areas of medicine. On the otherhand, pulse oximetry is prolific in critical care areas of a hospital,but is not found in vascular centers. The knowledge and understanding ofpulse oximetry technology (e.g., SpO₂ and PI) and its use in criticalcare, and of PPG technology and its application in vascular centers, areconcentrated in their respective specialties and are not shared. It isunexpected and surprising to use SpO₂ and PI for vascular testing, asdescribed herein, as application of such pulse oximetry parameters invascular testing requires knowledge and understanding of bothspecialties.

Returning to FIG. 2, in various embodiments, the pulse oximeter 18 isconfigured to process the electronic data received from the sensor 20and output pulse oximetry data to the processor 22. Such pulse oximetrydata includes, but is not limited to, perfusion index (PI) data. Invarious embodiments, pulse oximetry data may also include a volumetricpulse waveform and/or oxygen saturation (SpO₂) data. In otherembodiments, the pulse oximeter 18 may only provide SpO₂ and pulse ratedata. Some pulse oximeters may or may not provide a volumetric pulsewaveform or PI data to the processor 22. In such cases, PI data maypotentially be determined by the processor 22 based on the signals ordata received from the pulse oximeter 18. The processor 22 may also beconfigured to control operational aspects of the pulse oximeter 18 byissuing control signals to the pulse oximeter.

As mentioned above, in at least one embodiment, a pressure transducer 16directly connected to the pressure cuff 14 converts raw measurements ofcuff pressure to electronic pressure data that is automaticallymonitored by the processor 22. The pressure transducer 16 may providesuch pressure data continuously or intermittently to the processor 22.The monitored pressure data is used by the processor 22 in determiningthe systolic blood pressure of the patient's limb or digit.

In at least one embodiment, the processor 22 is an electronic controllerthat implements computer-executable algorithms, typically stored inmemory 24, and reads, processes, analyzes, calculates, and communicatesdata to an output 26. Memory 24 may comprise any form of anon-transitory computer-readable medium capable of storingcomputer-executable instructions that implement the algorithms describedherein. In at least one embodiment, the processor 22 is responsible forcontrollably inflating the pressure cuff 14 to either a pre-set pressureor to a pressure determined in relation to the cessation of thepatient's blood flow under the cuff 14.

In various embodiments, the processor 22 monitors the cuff pressuresensed by the pressure transducer 16 as well as data produced by thepulse oximeter 18 while inflating and deflating the pressure cuff 14. Inother embodiments, the processor 22 monitors the cuff pressure and thepulse oximeter data during inflation or deflation only. As describedherein, the processor 22 uses the pulse oximeter data (e.g., PI) to helpidentify systolic blood pressure events, and further uses the pressuredata to determine the patient's systolic blood pressure.

A systolic blood pressure event occurs when the processor 22 determinesthat the patient's flow of blood under the pressure cuff 14 has eitherceased or returned to flow. Systolic pressure represents systole or thepoint of peak contraction of the left ventricle of the heart. When theprocessor 22 determines that a systolic blood pressure event hasoccurred, the processor 22 captures the cuff pressure at that time andstores the cuff pressure in the memory 24. The stored cuff pressure maylater be used, alone or in combination with other systolic pressuremeasurements, to assess the peripheral vascular condition of the patient12. For example, the systolic pressure measurements may be used tocompute an assessment index for the patient, such as an ABI and/or TBI.Alternatively or in addition, the cuff pressure data may be transferredto the output 26.

In various embodiments, several systolic pressure measurements are usedto compute an ABI or TBI. While FIG. 2 illustrates only one pressurecuff 14, one transducer 16, one pulse oximeter 18, and one sensor 20, inother embodiments, the processor 22 may control and/or monitor two ormore pressure cuffs and transducers, as well as control and/or monitordata received from two or more pulse oximeters, separately orsimultaneously.

In some embodiments, the output 26 may be a data port to which variouselectronic devices may connect and receive data from the processor 22.In other embodiments, the output 26 may comprise one or more electronicdevices that provide data in a human-perceptible form, e.g., a printeror a display screen. Such data may include (but are not limited to)systolic pressure determinations as well as assessment index data asdiscussed herein. In addition to delivering such data to the output 26,the processor 22 may also store such data in memory 24 for laterretrieval.

FIG. 3 is a general flow diagram of algorithms and data that may be usedby a medical device's diagnostic apparatus, such as apparatus 10 in FIG.2, to assess a patient's peripheral vascular condition. As illustratedin FIG. 3, cuff pressure, volumetric pulse waveform, SpO₂, and PI data30 are read simultaneously by the processor 22 in real-time from thepressure transducer(s) 16 and pulse oximeter(s) 18. Since all of thedata is in electronic form, the processor 22 can receive this data via avariety of interfaces including (but not limited to) USB, RS232, andSerial Peripheral Interface, depending on the interface(s) available tothe processor 22.

Data processing 32 as well as control of peripherals such as thepressure cuff 34 occurs in the processor 22, which is the centralcontrol point for the diagnostic apparatus 10. While the processor 22monitors cuff pressure and pulse oximetry data, such as PI, volumetricpulse waveform, and SpO₂, the processor may simultaneously control thecuff pressure.

Cuff pressure is automatically controlled by one or more algorithmsimplemented in the processor 22. For example, a control algorithm 34implemented by the processor 22 may determine that the pressure in thepressure cuff 14 needs to decrease. Accordingly, the processor 22 mayissue a control signal to a pump attached to the cuff 14 (e.g., pump 16a in FIG. 4) that causes the pump to turn off. Another control signalwould cause a valve connected to the cuff (e.g., valve 16 b in FIG. 4)to open and allow air in the cuff to vent to the atmosphere at acontrolled rate. If the control algorithm 34 determines that thepressure in the pressure cuff 14 needs to increase, control signals areissued that cause the valve to close and the pump to turn on, so as toinflate the cuff. Should the control algorithm 34 determine to maintainpressure in the pressure cuff as the next step, control signals areissued that enable the valve to remain closed and the pump to turn off.In various embodiments, the control algorithm 34 for controlling thecuff pressure is subject to control by data processing algorithms 32executing in the processor 22, which controls the overall operation ofthe apparatus 10.

A patient's systolic blood pressure is determined by an algorithm 36that monitors signals and/or data received from the pulse oximeter 18,such as PI, SpO₂, and volumetric pulse waveform, and the pressureapplied by the cuff 14. Further detail regarding the processing that maybe implemented by the algorithm 36 for determining systolic pressure isdiscussed later herein.

Regarding SpO₂, it is noted that pulse oximeter technology has beenavailable and well understood for some time. Pulse oximeters haveprimarily been used to non-invasively measure the percentage of arterialhemoglobin saturated with oxygen (SpO₂). As local blood flow decreases,less oxygen is available to the tissue. When this occurs, SpO₂ decreasesand there is a negative slope to the SpO₂ data output by the pulseoximeter 18. Related to this scenario, SpO₂ increases with increasingblood flow, and when this occurs, there is a positive slope to the SpO₂data output by the pulse oximeter 18.

Perfusion index data indicates the variability of the patient's pulse asan index derived from the following classic pulse oximetry equation:

R(or Omega)=(ACred/DCred)/(ACinfrared/DCinfrared).

SpO₂ is determined as a function of R. The above fractions may begenerated using data from the volumetric pulse waveform. The ACredparameter is the dynamic, pulsatile component of the patient's pulse asilluminated by using a red LED in the pulse oximeter sensor 20, and theDCred parameter is the non-pulsatile component due to blood exterior tothe arterioles. The other LED typically used in a pulse oximeter sensor20 emits an infrared wavelength. The ACinfrared parameter is thedynamic, pulsatile component of the patient's pulse as illuminated byusing the infrared LED, and the DCinfrared parameter is thenon-pulsatile component due to blood exterior to the arterioles.Generally, the SpO₂ value is primarily a function of R obtained from acalibration curve that may be generated in a laboratory environment forthe pulse oximeter manufacturer.

The fraction in the denominator of R is considered to be the PI sinceoxygenated hemoglobin is more sensitive to infrared light than redlight. As blood flow decreases, the infrared pulsatile componentdecreases, and thus the PI decreases and there is a negative slope tothe PI data output by the pulse oximeter 18. Related to this scenario,PI increases with increasing blood flow, and in such circumstances,there is a positive slope to the PI data. PI thus reflects local changesin perfusion in the patient.

It is noted that the volumetric pulse waveform is a transducedrepresentation of a patient's pulse. As blood flow in the patientdecreases, the pulse amplitude decreases, and so as the volumetric pulsewaveform decreases, there would be a negative slope to the volumetricpulse waveform data output by the pulse oximeter. Related to thisscenario, volumetric pulse waveform increases with increasing bloodflow, and in such circumstances, there is a positive slope to thevolumetric pulse waveform data. The volumetric pulse waveform thusreflects local changes in perfusion in the patient.

As discussed earlier, an assessment of a patient's peripheral vascularcondition may utilize a device, such as apparatus 10, having at leastone pressure cuff 14 and at least one pulse oximeter 18. In many cases,the assessment involves two or more cuffs 14 and two or more pulseoximeters 18 being monitored simultaneously. Alternatively, anassessment with two or more cuffs and two or more pulse oximeters may beconducted serially. In either case, when a systolic blood pressure eventoccurs as determined by the processor 22, the algorithm 36 captures thecuff pressure at the time of the event, and possibly stores the pressuremeasurement in memory 24.

When the algorithm 36 has determined all required systolic pressuremeasurements, another algorithm 38 operating in the processor 22 mayautomatically calculate an assessment index, such as an ABI and/or TBI,using the systolic pressure measurements. Further details regarding theprocessing steps that may be implemented by the algorithm 38 forcalculating an ABI and/or TBI are discussed below with regard to FIG. 5.

FIG. 4 is a detailed block diagram of the apparatus 10 illustrated inFIG. 2. As indicated, a patient 12 is fitted with a pressure cuff 14 ona limb or digit. The pressure cuff 14 may be any one of a vascular orstandard blood pressure cuff that is wrapped around the limb or digit tocontrol blood flow in the limb or digit during a pressure measurement. Avascular cuff is typically constructed such that a bladder containingair completely surrounds the limb or digit. A vascular cuff isconstructed so that the bladder is long enough to overlap when wrappedaround the limb or digit in order to simultaneously compress all of thesoft tissue under the cuff. Also, the cuff is constructed with properwidth to avoid obtaining a pressure that is higher or lower thanexpected. When a systolic pressure event is detected as discussed below,and the patient's systolic pressure is determined, the pressuremeasurement represents the patient's systolic blood pressure at the siteof the pressure cuff 14, not distal to the cuff site.

A pulse oximeter sensor 20 is placed on the limb or digit of the patienton the same side of the patient's body (left or right) as the cuff 14.The pulse oximeter sensor 20 uses at least two LEDs of differentwavelengths to detect the volumetric pulse waveform in the patient'slimb or digit. As mentioned earlier, there are two typical sensordesigns for a pulse oximeter sensor. A transmission-based sensortransmits LED light through a limb or digit on one side, and uses aphotodiode placed on the other side of the limb or digit to detect thetransmitted light that passes through the limb or digit. Areflectance-based sensor has both the LEDs and a photodiode located onthe same side of the limb or digit. Such a sensor transmits LED lightinto the tissue of the limb or digit, and detects light that has passedthrough the tissue and is reflected back to the sensor. In either case,signals representative of the light detected by the photodiode areprovided to the pulse oximeter 18.

The pulse oximeter 18 monitors the signals provided by the sensor 20,processes the signals, and generates data, such as perfusion index (PI),volumetric pulse waveform, and oxygen saturation (SpO₂) for furtherevaluation. Processes for calculating and using PI data are known in theart. For example, European Patent Application No. 1861000 B1, titled“Method and device for determining the perfusion of blood in a bodymember” and incorporated by reference herein, discloses suitableprocesses that may be employed by the pulse oximeter 18. As anotherexample, an article by Lima et al., published in Critical Care Medicine,2002 June; 30(6):1210-3, titled “Use of a peripheral perfusion indexderived from the pulse oximetry signal as a noninvasive indicator ofperfusion” and incorporated by reference herein, also disclosesprocesses for using PI.

Signals 40 from the pulse oximeter 18 are available to the processor 22for further processing as described above. The data may be stored in adata store, such as memory 24. The data also may be output for display(e.g., using display device 26 a), for communication to another device(e.g., using communication port 26 b), or for printing (e.g., usingprinter 26 c).

A pressure transducer 16 detects the pressure in the pressure cuff 14and provides a pressure measurement signal 42 in a form that theprocessor 22 can read. The pressure signal provides the data necessaryfor the processor 22 to not only control the pressure in the pressurecuff 14, but also determine the patient's systolic pressure via one ormore algorithms implemented by the processor 22. In various embodiments,the pressure transducer 16 is directly attached to the cuff 14.

The apparatus 10 includes a pump 16 a, which is an electromechanicaldevice that increases the pressure in the cuff 14 based on a pumpcontrol signal 44 received from the processor 22. The pump controlsignal 44 is determined by one or more algorithms operating in theprocessor 22. In various embodiments, the pump 16 a is directly attachedto the pressure cuff 14, and turns ON and OFF on demand under thecontrol of the processor 22.

The apparatus 10 further includes a valve 16 b, which is anelectromechanical device that allows pressure in the pressure cuff todecrease based on a valve control signal 46 received from the processor22. The valve control signal 46 is determined by one or more algorithmsoperating in the processor 22. In various embodiments, the valve 16 b isdirectly attached to the pressure cuff 14, and opens and closes ondemand under the control of the processor 22. When the valve 16 b isopened, pressurized air within the cuff 14 is released to theatmosphere, thus allowing the cuff pressure to decrease.

In various embodiments, the pump control signal 44 may control the speedor volume of the air pumped into the cuff 14 to control the rate atwhich the cuff is inflated. Likewise, the valve control signal 46 mayalso control the degree to which or time that the valve 16 b is opened,thus controlling the rate at which pressurized air within the cuff 14 isreleased. Through control of the pump 16 a and the valve 16 b, theprocessor 22 is able to controllably apply pressure via the cuff 14 tothe limb or digit of the patient.

In various embodiments, the processor 22 may be an integrated circuitthat operates according to computer-executable software instructions.The processor 22 is configured to read, process, analyze, interpret, andcommunicate data based on algorithms implemented by executing thesoftware instructions. The processor 22 essentially comprises theintelligence of the apparatus 10 in the sense of command, control,analysis, and communication. It allows data to be processed, devices tobe controlled simultaneously, and the process for assessing the vascularcondition of the patient to be accomplished automatically.

In the illustrated embodiment, data obtained and processed by theprocessor 22 includes pressure data from the pressure transducer 16 andpulse oximeter data (e.g., PI, SpO₂, and volumetric pulse waveform) fromthe pulse oximeter 18. Based on this data, the processor 22 determinesthe patient's systolic pressure and calculates an assessment index, suchas an ABI and/or TBI.

In various embodiments, data storage takes place inside an electricalstorage device, such as memory 24, in which data may be temporarily orpermanently stored for a desired amount of time. Examples of such astorage device may include but are not limited to non-volatile memorysuch as flash, or volatile memory such as RAM. The storage device may befixed within the apparatus 10 or it may be removable from the apparatus10.

Data display 26 a occurs in the form of numerics and/or graphicsgenerated on a screen as instructed by the processor 22. Typically, thesubject matter displayed depends on the measurement parameter. Displayscreens may include, without limitation, standard computer monitors ortouch LCD screens, and screens such as those found in smart phones,tablet computers, and other handheld wireless devices.

Data communications 26 b are provided in accordance with a protocol bywhich data is transferred from the processor 22 or from data storage(e.g., memory 24) to another location. For example, and withoutlimitation, DICOM, HL7, and other protocols may be used. Wired as wellas wireless communication options are possible, as well as USB andInternet communications.

Data printing 26 c may occur via a protocol by which data is sent to aprinter. Wired as well as wireless printers operating over a wirelessnetwork are options.

FIG. 5 is a detailed flow diagram of algorithms that may be implementedby the diagnostic apparatus 10 described herein. As indicated, cuffpressure as well as PI, volumetric pulse waveform, and SpO₂ data fromthe pulse oximeter are read 50 by the processor. The data may bepre-processed 54 before or after receipt by the processor. For example,the data may be filtered to reject noise and spurious signals. Filteringmay be performed in the time or frequency domains, but in either case,it is desirable that the filtering be fast and efficient to accommodatereal-time diagnostic systems. The filtering performed 54 may occurthroughout the measurement cycle.

In the illustrated embodiment, cuff pressure is automatically controlled52 throughout the measurement cycle. Control of the cuff pressure isaccomplished quickly so that the systolic pressure measurementscorrespond with the changes in PI, volumetric pulse waveform, and/orSpO₂ in the patient, and are thus indicative of a cessation of bloodflow or a return of blood flow under the cuff.

After pre-processing, data are further processed 56 during inflation ofthe cuff in accordance with algorithms that monitor the cuff pressureand simultaneously detect changes in the pulse oximeter parameters, suchas PI, volumetric pulse waveform, and SpO₂. For example, for eachparameter, an algorithm interprets 58 the nature of change in theparameter and from that information, determines a systolic bloodpressure 60 when a systolic pressure event has occurred (i.e., duringinflation, when blood flow under the cuff has ceased). During inflation,PI, SpO₂, and volumetric pulse waveform data are expected to decrease,and when blood flow under the cuff has ceased, the data are expected tolevel out in that the rate of change is very small. In variousembodiments, it is in the region of leveling that the instantaneous cuffpressure is captured and interpreted as the patient's systolic pressure.A decrease in PI values, for example, thus indicates the cessation ofblood flow in the limb or digit to which the cuff is applied.

During deflation of the cuff, data are processed 62 in accordance withalgorithms that monitor cuff pressure and simultaneously detect changesin the pulse oximeter parameters, such as PI, volumetric pulse waveform,and SpO₂. During deflation, the respective pulse oximeter parameters areexpected to increase. For each parameter, an algorithm interprets 64 thenature of change in the parameter and from that information, determinesa systolic blood pressure 66 when a systolic pressure event has occurred(i.e., when blood flow under the cuff has returned). In this scenario,it is in the region in which the parameter values turn upward that theinstantaneous cuff pressure is captured and interpreted as the patient'ssystolic pressure. An increase in PI values, for example, thus indicatesthe return of blood flow in the limb or digit to which the cuff isapplied.

After determining the patient's systolic blood pressure using one ormore parameters of pulse oximeter data (e.g., PI, SpO₂, and/orvolumetric pulse waveform), the pressure determination may be validated68. By validating the pressure determination, the diagnostic apparatus10 is better able to assure the clinician that the reported measurementmeets specific criteria and is therefore reliable as a clinicalmeasurement. In at least one embodiment, to be a valid clinicalmeasurement, the determined systolic pressure should fall within aspecific pressure range, as well as correlate with respect to SpO₂ andPI values obtained. This determination may be accomplished automaticallyat the end of each systolic pressure measurement. In some circumstances,the pressure measurement reported by apparatus 10 may be validatedthrough correlation with data obtained using other devices, such as aDoppler instrument.

As mentioned earlier, the measurement process involves at least one cuff14 and at least one pulse oximeter 18. Other configurations arepossible. For example, depending on the configuration desired, it may beappropriate to measure six pressures, namely two brachial pressures, twodorsalis pedis pressures, and two posterior tibial pressures,automatically and simultaneously using two brachial and two ankle cuffs,with pulse oximeter sensors on relevant fingers and toes.

When the apparatus 10 has determined the systolic blood pressuremeasurements and the systolic blood pressure measurements have beenvalidated, the apparatus may calculate 70 an assessment index for thepatient, such as an ABI and/or TBI. The ABI may be determined as thehighest of the ankle pressures on a given leg divided by the higher ofthe two brachial pressures. The TBI may be determined as the toepressure divided by the highest of the brachial pressures. If thepatient has two legs, he will have two ABI measurements calculated.

Once the ABI and/or TBI calculation has been performed, the assessmentindex may be validated 72 or correlated, e.g., in a manner as discussedearlier, with the SpO₂ and PI measurements or other measurement devices,to assure accuracy.

Informal testing has shown a good correlation between pulseoximeter-based systolic blood pressure measurements as described herein,and Doppler-based systolic blood pressure measurements. Doppler-basedsystolic blood pressure measurements of four subjects were obtained andcompared with systolic blood pressure measurements obtained fromobserving oxygen saturation (SpO₂), perfusion index (PI), and volumetricpulse waveform data as displayed by a commercially available pulseoximeter. The commercially available pulse oximeter that was utilized isbuilt and manufactured for medical device manufacturers to incorporateinto their devices.

Results showed generally good correlation between Doppler-based andpulse oximeter-based systolic blood pressure measurements (within +/−5mmHg) determined on inflation as well as deflation of the pressure cuff.The pulse oximeter is able to provide accurate pulse oximeter values atlow perfusion and in the presence of patient motion.

As can be seen above, the algorithmic use of PI, SpO₂, and thevolumetric pulse waveform allows for a more accurate determination ofsystolic blood pressure. The procedures discussed above also enableautomatic determination of systolic blood pressures from potentiallymultiple pulse oximeters simultaneously, without using Doppler or PPG.Furthermore, numeric correlation of SpO₂, PI, and volumetric pulsewaveform can be used to validate the resulting systolic pressuremeasurement.

While various representative embodiments, modes of operation, andprinciples of the invention have been described in the foregoingdescription, aspects of the invention that are protected by thefollowing claims should not be construed as limited to the particularembodiments disclosed. The embodiments described herein are to beregarded as illustrative rather than restrictive. It will be appreciatedthat variations and changes may be made therein, and equivalentsemployed, without departing from the spirit of the present disclosure.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of assessing aperipheral vascular condition of a patient, the method comprising :applying pressure to a limb or digit of the patient; using a pulseoximeter distal to the applied pressure to generate a plurality of pulseoximeter parameters relative to the limb or digit of the patient,wherein the plurality of pulse oximeter parameters comprises avolumetric pulse waveform and a perfusion index slope; while increasingor decreasing the pressure applied to the limb or digit, determining achange in each of the plurality of pulse oximeter parameters indicativeof a change of blood flow in the limb or digit; and assessing theperipheral vascular condition of the patient based on blood flow in thelimb or digit as determined by the change in each of the pulse oximeterparameters.
 2. The method of claim 1, wherein applying pressure to thelimb or digit of the patient comprises using a pressure cuff to compressthe limb or digit.
 3. The method of claim 2, wherein the pressure cuffinflates or deflates at a controlled rate.
 4. The method of claim 1,wherein using the pulse oximeter to generate the plurality of pulseoximeter parameters comprises: transmitting light into the limb ordigit; detecting light that was transmitted through or reflected fromtissue in the limb or digit; and calculating the plurality of pulseoximeter parameters relative to the limb or digit based on the lighttransmitted through the tissue.
 5. The method of claim 1, whereindetermining the change in the plurality of pulse oximeter parameterscomprises identifying when a decrease in at least one of the pluralityof pulse oximeter parameters occurs, in which the decrease is indicativeof a change of blood flow in the limb or digit.
 6. The method of claim1, wherein determining the change in the plurality of pulse oximeterparameters comprises identifying when an increase in at least one of theplurality of pulse oximeter parameters occurs, in which the increase isindicative of a change of blood flow in the limb or digit.
 7. The methodof claim 1, further comprising calculating an assessment index for thepatient by comparing a peripheral vascular condition determined in anankle of the patient to a peripheral vascular condition determined in anarm of the patient.
 8. The method of claim 1, further comprisingcalculating an assessment index for the patient by comparing aperipheral vascular condition determined in a toe of the patient by aperipheral vascular condition determined in an arm of the patient. 9.The method of claim 1, wherein assessing the peripheral vascularcondition of the patient based on the systolic blood pressure determinedin the limb or digit comprises using systolic blood pressure determinedin both an ankle and a toe of the patient.
 10. The method of claim 1,wherein the plurality of pulse oximeter parameters further comprises aperfusion index and an oxygen saturation.
 11. The method of claim 1,wherein the method is under the control of a computing device.
 12. Anapparatus for assessing a peripheral vascular condition of a patient,the apparatus comprising: a pressure cuff configured to apply pressureto a limb or digit of the patient; a pulse oximeter configured togenerate a plurality of pulse oximeter parameters of the patient at aposition on the limb or digit distal to the pressure cuff, wherein theplurality of pulse oximeter parameters comprises a volumetric pulsewaveform and a perfusion index slope; and a processor configured to:cause the pressure cuff to controllably apply or release a pressure tothe limb or digit of the patient sufficient to change blood flow to thelimb or digit distal to the pressure cuff, monitor the plurality ofpulse oximeter parameters generated by the pulse oximeter relative tothe limb or digit and determine a change in each of the plurality ofpulse oximeter parameters indicative of the change of blood flow in thelimb or digit, and determine a systolic blood pressure in the limb ordigit based on the pressure applied by the pressure cuff when the changeof blood flow occurred as determined by the change in each of theplurality of pulse oximeter parameters.
 13. The apparatus of claim 12,wherein the pressure cuff is configured to apply pressure to the limb ordigit by inflating and thereby compressing the limb or digit, andwherein the processor is configured to cause the pressure cuff tocontrollably apply or release a pressure by inflating or deflating thepressure cuff at a controlled rate.
 14. The apparatus of claim 12,further comprising one or more optical devices configured to transmitand detect light when attached to the patient, wherein the plurality ofpulse oximeter parameters are generated by the pulse oximeter using theone or more optical devices.
 15. The apparatus of claim 14, wherein thepulse oximeter is configured to transmit light into or onto the limb ordigit and detect light transmitted through or reflected from tissue inthe limb or digit, and wherein the processor is configured to monitorthe plurality of pulse oximeter parameters by the pulse oximeter basedon the light transmitted through or reflected from the tissue.
 16. Theapparatus of claim 15, wherein the pulse oximeter comprises an opticalsource configured to transmit light at specific wavelengths and aphotodiode configured to detect light transmitted at the specificwavelengths.
 17. The apparatus of claim 12, wherein the plurality ofpulse oximeter parameters further comprises a perfusion index and anoxygen saturation.
 18. The apparatus of claim 12, wherein the processoris further configured to calculate an assessment index for the patientby comparing a peripheral vascular condition determined in an ankle ofthe patient to a peripheral vascular condition determined in an arm ofthe patient.
 19. The apparatus of claim 12, wherein the processor isfurther configured to calculate an assessment index for the patient bycomparing a peripheral vascular condition determined in a toe of thepatient to a peripheral vascular condition determined in an arm of thepatient.